SAMPE Journal - May/June 2016 - 52

Feature Article

Figure 10 (a) and (b). Stable end crushing failure.

(a) Front view

(b) Isometric view

Table 4. Adhesive bond fracture toughness test results.

The critical strain energy release rate, GC, can be
expressed as,
[1]

where, P is the external load; dδ is the displacement;
B is the width of specimen and Da is the incremental
crack length during the process of test. The energy
required for crack growth, òPdd,can be calculated by
calculating the area enclosed by the loading-unloading
path. The area under the loading/unloading path
is obtained by integration using the trapezoid rule.
The average interface fracture energy release rate
for samples fabricated under full vacuum and partial
vacuum is shown in Table 4. About 10% variation was
noticed in samples fabricated under full vacuum and
partial vacuum.

Summary and Conclusions
High temperature sandwich composite panels were
manufactured using the OOA prepreg process. Two
levels of vacuum pressure were studied, full vacuum
(100 kPa) and partial vacuum (80 kPa). The prepreg
cure temperature were used to manufacture the
panels. No warping was visible after post cure at 232°C
(450°F), which implies that the panels had sufficient
green strength to enable a free-standing post-cure.
Outgassing of adhesive during the cure cycle was
studied using TGA. A mass drop of 1.7% was observed
during the ramp and cure isotherm. These volatiles
need to be removed during cure to avoid foaming
issues. Adhesive bond strength was evaluated using
52

FWT tests and adhesive bond fracture toughness
tests. The samples manufactured using full vacuum
pressure during cure exhibited in a 6-8.5% increase
in FWT strength compared to those manufactured
at a partial vacuum pressure. FWT strength reduced
when samples were tested at 177°C (350°F). Cohesive
failure dominated in all samples while some regions
of adhesive failure were also present. Sandwich
samples with [0°/90°]2s facesheet orientation were
fabricated and a modified CSB test was conducted
at room temperature. Samples exhibited a strain
energy release rate of 335.19 J/m2 (full vacuum) and
301.80 J/m2 (partial vacuum). Edgewise compression
tests were conducted on samples with facesheet
orientation [0°/90°]s. Vacuum pressure levels used
in this study, did not have a significant effect on
edgewise compressive strength. Increase in test
temperature to 177°C (350°F) resulted in a reduction
in edgewise compressive strength. This can be a result
of reduced core stiffness at increased temperatures.
Measured FWT strengths at room temperature were
comparable to those mentioned in previous studies
involving aerospace grade epoxy adhesives.
Acknowledgements
This research is sponsored by the Industrial
Consortium of the Center for Aerospace Manufacturing
Technologies (CAMT) at Missouri University of Science
and Technology. The authors would like to thank
Stratton Composite Solutions and Cytec Engineered
Materials for the materials supplied.
SAMPE Journal, Volume 52, No. 3, May/June 2016
Table of Contents for the Digital Edition of SAMPE Journal - May/June 2016
